The death of a 3-month-old girl who contracted the flu last year might never have an official explanation.

Larimer County health officials had previously said the baby possibly died from the flu in November 2017, but the coroner’s office could not confirm the information. The Coloradoan obtained a summary of the girl’s autopsy report this week.

“Despite a thorough death investigation, full autopsy and ancillary studies, a specific cause of death could not be identified,” stated the report, dated Feb. 5.

An initial screening test for flu was positive, but two other tests yielded negative results, according to the report. The report may be amended if more information becomes available.

The baby’s manner of death is logged as “undetermined,” and her cause of death is “sudden unexplained infant death.”

Unexplained deaths are rare in Larimer County. Of about 1,200 deaths that fell under medical examiner jurisdiction last year, the coroner found six to be of undetermined manner. The medical examiner reviewed less than half of about 2,800 deaths in Larimer County in 2017.

Colorado has recorded one pediatric flu death and about 4,600 hospitalizations this season, according to state health data. That’s the highest number of hospitalizations since they became a reportable condition in 2004-05.

The hospitalization spike reflects a national trend of unclear origins, according to the Centers for Disease Control and Prevention.